(1) Field of the Invention
The present invention relates to methods for preparing lyophilized pellets of biological materials such as proteins, in particular to methods of preparing lyophilized pellets of biological materials that are spherical in shape and have fast reconstitution times.
(2) Description of Related Art
Biological materials such as cells, proteins and vaccines are frequently preserved by lyophilizing aliquots of a liquid composition containing the biological material. The lyophilization process involves freezing a liquid sample which is then subjected to a vacuum so that the ice in the frozen sample directly changes to water vapour or sublimes. After the removal of ice, the sample temperature is gradually increased (while still under vacuum) and water is desorbed from the remaining non-ice phase of the sample.
Lyophilized cakes of a biological material are prepared by aliquoting into a glass container a desired amount of the biological material, which is typically present in a buffered solution with appropriate stabilizers (i.e., a “formulation”) and then subjecting the glass container containing the biological material to steps of cooling, freezing, annealing, primary drying and secondary drying. The glass container containing the dried biological material is typically stored for long periods of time at room temperature or under refrigerated conditions. The dried formulation containing the biological material is typically reconstituted by adding a liquid, usually water, to the glass container. Glass containers used for lyophilizing biological materials intended for use as therapeutics and vaccines typically have included glass vials and dual chamber injection devices, in which one chamber contains the lyophilized cake and the other chamber contains the reconstituting liquid.
Methods of lyophilizing biological materials in the form of spherically shaped pellets, i.e., beads, have also been described. In these methods, individual samples of the biological material are frozen and dried prior to placing a desired number of the dried samples into a storage container such as a glass vial. Historically, these methods relied on either (a) dispensing an aliquot of a liquid composition containing the desired amount of a biological material into a container of a cryogen such as liquid nitrogen, which results in direct contact of the biological material with the cryogen and/or (b) dispensing an aliquot of a liquid composition containing the biological material into a cavity present on a chilled solid plate, where the cavity contains the aliquot until it is frozen. It should also be noted that the use of plates with machined cavities often requires use of an automated system for detachment of the pellets from the cavity wall. Furthermore, reliance on a cavity to contain the liquid aliquot results in a volume restriction on the size of the aliquot and resulting pellet. Another approach, which is referred to as the die and punch method and uses a closed mould and compressive force to obtain a frozen pellet, suffers from a complex assembly design, leakage of fluid formation from the cavity and sticking of pellet to either the die or the punch.